Quadrature amplitude modulation with 16 levels (16-QAM) is one of the candidate modulation formats for 100 Gigabit/s (Gbit/s) transmission into optical fibre. It encodes four bits on a constellation of sixteen points, with four different amplitude values of both the in-phase and quadrature components of the transmitted signal. When considering a polarization-diversity 16-QAM format for the transmission of 100 Gbit/s, the required symbol-rate for the transmitter is 12.5 Gbaud. Even at this reduced symbol-rate, the generation of multi-level driving signals can be complicated. Further complication arises from the necessity of applying differential digital encoding to the optical signal before optical modulation, to solve the π/2 phase ambiguity of the QAM constellation which would otherwise arise when estimating the carrier phase at the receiver.
There are four known 16-QAM transmitter/modulator schemes, as follows. The first comprises a conventional in-phase/quadrature (I-Q) modulator in which each of the in-phase and quadrature components is a four-level signal that can be obtained by using a four-level driving voltage. Despite the simplicity of the optical scheme, the requirement of the generation of four-level driving voltages makes the transmitter less attractive for the implementation of 100 Gbit/s systems.
A second scheme comprises a single dual-drive Mach Zehnder modulator (MZM), as reported in K.-P. Ho and H.-W. Cuei, “Generation of arbitrary quadrature signals using one dual-drive modulator,” J. Lightwave Technol., vol. 23, no. 2, pp. 764-770, February 2005. The output signal can assume any value in the complex plane by properly choosing the driving voltages for the two MZMs. This scheme features the simplest optical components, however, the generation of the 16-QAM constellation requires a very complex driving-voltage scheme with up to 16-level signals.
A third 16-QAM transmitter structure, comprising a phase and amplitude I-Q modulator, requiring only two-level driving voltages, has been reported in M. Seimetz, “Multi-format transmitters for coherent optical M-PSK and M-QAM transmission”, in Proc. ICTON'05, 2005, pp. 225-229, paper Th.B1.5. The basic structure is similar to a conventional I-Q modulator but each branch also comprises a phase modulator. In each arm, the MZM generates the two amplitude levels {⅓, 1}, and the phase modulator (PM) sets the phase to zero or π, to obtain the required four-level signals in each quadrant of the constellation diagram.
The fourth 16-QAM transmitter structure comprises two quaternary phase shift keying (QPSK) modulators nested inside a Mach-Zehnder interferometer having an 80:20 output combining ratio, as reported in J. M. Kahn and E. Ip, “Carrier synchronization for 3- and 4-bit-per-symbol optical transmission”, J. Lightwave Technol., vol. 23, no. 12, pp. 4110-4114, December 2005. This scheme achieves a 16-QAM modulation by using binary driving signals for each QPSK modulator. However, it still requires electrical processing of the input bit sequence to apply quadrant differential encoding.
Lower and higher order QAM, such as quaternary amplitude modulation (4-QAM) and 64-QAM, are also known for digital data encoding, and will be referred to herein collectively as 2n-QAM.